This is normal superheterodyne radio receiver. It does not use any coils. It uses a filter and oscillator made by OP-AMP. Converter is also constructed by a OP-AMP detector.
When the plus input is positive , it pass through the antenna signal. When the plus input is negative , it does not pass through the antenna signal. Therefore antenna signal is converted to 40kHz IF signal. IF filter is constructed by the low pass filter. Q of it is higher than 1. So it works as a band pass filter. See fig 4! IF-AMP is a normal non-inverted-AMP. Detector is OP-AMP detector. You can use a normal diode detector indicated in fig 3. But I wanted to use OP-AMP also in this block. Low pass filter selects only audio signal. The output impedance of OP-AMP is about 1k ohm. Therefore it can not drive a speaker directly. So I use a transformer. But now I think it is better to use a emitter follower indicated on another page. I can hear JOAK(594kHz) and JOAB(693kHz) by tuning the oscillator. This radio has the total gain 4+23+29+4+17=77dB. 1m wire is enough antenna.
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